(1) Field of the Invention
This invention relates broadly to apparatus for spraying onto a substrate or workpiece molten or heat-softened material at high velocities by means of hot combustion gases.
More particularly, this invention relates to an improved combustion head for feeding hot combustion gases under pressure and solid material, which may be metal wire or metal powder, to the inlet of the nozzle of a flame spray apparatus, which material, exiting the nozzle in the molten or heat softened state, is projected at high velocity onto the surface of a substrate or workpiece, thereby to provide such substrate or workpiece with a dense and tightly adherent coating of said material.
(2) Description of the Prior Art
Flame spraying apparatus operated by hot combustion gases for coating substrates or workpieces with metal is well-known in the art, as shown, for example, in U.S. Pat. Nos. 4,342,551 (1982), 4,343,605 (1982), 4,416,421 (1983), 4,568,019 (1986) and 4,634,611 (1987).
U.S. Pat. No. 4,416,421 (1983) is particularly pertinent to the present invention and discloses high velocity flame spray apparatus comprising a combustion chamber receiving at one end thereof a continuous supply of pressurized fuel, which may be liquid or gas, through a first conduit, and a continuous supply of pressurized oxygen through a second conduit. Means, such as a spark plug, is provided to ignite the fuel. A continuous flow of highly pressurized hot combustion gases resulting from ignition of the fuel is discharged from the opposite end of the combustion chamber through a plurality of ports leading to the inlet of a relatively long nozzle having a converging throat. The longitudinal axis of the nozzle is perpendicular to the longitudinal axis of the combustion chamber, so that material, in powder form or in the form of wire, may be fed into the throat of the nozzle along the longitudinal axis of the nozzle. The ports conveying the highly pressurized combustion gases from the combustion chamber to the inlet of the nozzle are radially spaced around and are inclined toward the longitudinal axis of the nozzle so that the hot combustion gases, while propelling the material through the nozzle and in the course of doing so melting or at least heat-softening the material, restricts the flow of material through the nozzle to a thin column which does not touch the nozzle bore wall. The molten or heat-softened material exits the nozzle at high velocity and is projected onto the surface of a substrate or workpiece thereby to provide the said substrate or workpiece with a dense tightly-adhering coating of said material. Passageways for cooling water are provided around the exterior of the nozzle.
Flame spraying apparatus such as the apparatus disclosed in U.S. Pat. No. 4,416,421, operated in the manner known to the art, provides the substrate or workpiece with a coating of material having desired properties, such as abrasion resistance or corrosion resistance, lacking in the substrate or workpiece. The material constituting the coating may, for example, be far more expensive than the substrate or workpiece, and therefore considerable economies are realized by this process, in comparison with the cost of making the entire body of the substrate or workpiece of expensive material with the desired properties.
In a commercial embodiment of the flame spray apparatus disclosed in U.S. Pat. No. 4,416,421, which commercial embodiment has been used industrially in the United States and elsewhere for several years, and which is described in some detail further on in this specification, a combustion head is provided as a transition piece between the discharge end of the combustion chamber and the inlet end of the nozzle. The combustion head has a counterbore which receives the hot combustion gases continuously generated in the combustion chamber, and also is provided with a complex series of passages through which cooling water is passed. It has been observed that, in the combustion head currently in commercial use, the floor of the counterbore receiving the hot combustion gases from the combustion chamber cracks and erodes after a short period of use, thus severely limiting the useful life of this combustor head. Such cracking and erosion occurs, apparently, because of the development of local hot spots in the counterbore and thermal fatigue in the body of the combustion head adjacent to the counterbore. It is believed that cracks form, not because of the metallurgy of the combustor head, but because of the design of this counterbore.